FIG. 3 is a cross-sectional view of conventional example 1 of loudspeaker. This loudspeaker has a typical conventional structure, i.e., a single cone loudspeaker. As shown in FIG. 3, magnet 24 is bonded to yoke 25. Top plate 23 is bonded onto an upper surface of magnet 24. Yoke 25, magnet 24 and top plate 23 which are bonded to each other constitute magnetic circuit 22 having a magnetic gap having a cylindrical shape.
Magnetic circuit 22 is bonded to a lower surface of frame 21 made of resin. An outer periphery of cone diaphragm 28 is bonded to an outer periphery of frame 21. Voice coil 26 is bonded to a center part of cone diaphragm 28, and is placed in the magnetic gap formed in magnetic circuit 22. Damper 27 is bonded as to support voice coil 26. Dust cap 29 serving as a dustproof is placed at a center part of cone diaphragm 28
Patent Documents 1 and 2 are known as prior art documents related to the present invention.
This loudspeaker produces a phase difference between sounds generated at an inner part and an outer part of the cone diaphragm due to the difference between depths of these parts. The deeper is the cone diaphragm, the larger becomes the harmful influence, therefore deteriorating sound quality even if having preferable frequency characteristics.
FIG. 4 is a cross-sectional view of conventional example 2 of loudspeaker addressing this problem. In FIG. 4, components identical to those of conventional loudspeaker shown in FIG. 3 are denoted by the same reference numerals, and their detailed explanation will be omitted. This conventional speaker includes diffuser 30 attached to an upper surface of top plate 23. Diffuser 30 causes reflection and diffraction to shift the phase of the sound generated in the inner part of the cone diaphragm to that of the outer peripheral part.
However, conventional example 2 of loudspeaker which does not include a dust cap has a small vibrating area, thus reducing a sound pressure. Conventional example 2 of loudspeaker includes voice coil 26 having an upper part not sealed and reducing a dumping at low frequencies. In this loudspeaker, since a relative position between cone diaphragm 28 and diffuser 30 changes according to the amplitude of the sound, and may modulate phases.
FIG. 5 is a cross-sectional view of conventional example 3 of loudspeaker. In FIG. 5, components identical to those of conventional example 1 of loudspeaker shown in FIG. 3 are denoted by the same reference numerals, and their explanation will be omitted. Conventional example 3 of loudspeaker includes sub-cone 31 provided at an upper end of voice coil 26. Example 3 of loudspeaker causes reflection and diffraction, however, a concave space inside sub-cone 31 disturbs the phases, not improving the phase against the vibrating surface.
FIG. 6 is a cross-sectional view of a conventional example 4 of loudspeaker. Conventional example 4 of loudspeaker includes dust cap 42 fixed to an upper end of sub-cone 41 attached to the voice coil. This loudspeaker solves a part of the above mentioned problem. However, a sound generated by dust cap 42 and a sound generated by the sub-cone paper cause a phase interference with a difference in a distance between the dust cap and the cone paper, causing a large dip of a sound pressure at a certain frequency, still providing a problem.
Patent Document 1: JP63-52375U
Patent Document 2: EP1771035A